(a) Field of the invention
The present invention relates to a method of focussing and a photographic lens system therefor.
(b) Description of the prior art
In order to shorten the total length of a lens system for use in a compact camera, there are several methods available, one is to shorten a focal length of the lens system and another is to reduce the proportion between the length from the first surface of the lens system and the film surface to the focal length, that is, the telephoto ratio. In order to shorten the total length of the lens system markedly, it is necessary to use both these methods at the same time. Thus, it is desirable to use a lens system of telephoto type for the purposes of making the telephoto ratio small. When the focal length of the lens system of telephoto type is shortened, its angle of view becomes large so that it is necessary to make the lens characteristics good over a large angle of view at the off-axis.
Even if a photographic lens system satisfying the above-mentioned demands can be designed, that is, its total length is very short but the aberrations are well-corrected over a large angle of view, there exists a problem concerning the focussing. That is, in a lens system having the above-described composition, the back focus unavoidably becomes short so that it will be very difficult to arrange a shutter behind the lens system. Consequently, the shutter must be disposed between a front convergent lens group and a rear divergent lens group of the lens system of telephoto type. When the shutter is thus arranged between the front and rear lens groups, focussing by way of drawing out the total lens system becomes impossible. And, it has been known to make possible the focussing by way of fixing the front lens group and moving the rear lens group (as described, for example, in Japanese Published Unexamined Patent Application Nos. 139732/75 and 117126/77), however, by adopting this method, there must be a space in which the rear lens group moves, so that it becomes necessary to provide large spaces between the rear lens group and the film surface and between the shutter and the rear lens group and, as a result, the total length of the lens system becomes unavoidably long.